PT

PT 2

The nervous system has been compared with a computer system, with the brain acting as the central processing unit, relaying messages by way of the spinal cord through nerve fibers

◦ Nerves radiate to every structure in the body to provide connection for input and output data Myelinated nerves – have a coat of white fatty

material, interrupted along the length of the nerve at regularly spaced intervals -found mostly in the CNS

Nonmyelinated nerves – have a thin coat of myelin – found in the autonomic nervous system

Normally, impulses pass in only one direction.◦ Sensory neurons conduct impulses from the sense

organs to the spinal cord and brain◦ Motor neurons conduct impulses from the brain and

spinal cord to muscles and glands◦ Interneurons are responsible for delivering a signal

from one neuron to another The point at which an impulse is transmitted

from the axon of one neuron to the dendrite of another is a microscopic space called a synapse.

Myelinated nerves have a faster signal than nonmyelinated

There is no physical contact of neurons in the synapse.◦ The electrical impulses from one neuron to the

next neuron do not directly jump the synapse but instead cause release of a chemical at the synapse.

Neurons are so specialized that they have lost the power to produce new cells.◦ When an axon is severed, that part distal to the

cell body dies. Once a connection is broken within the CNS, it is

broken forever.

The CNS includes the brain and spinal cord◦ Each are protected from injury by the skeletal

system Brain – skull Spinal Cord – vertebrae

The CNS contains both white and grey matter◦ White color is created by myelinated fibers

formed by the bundles of axons and dendrites◦ Gray color is created by the masses of nerve cells

The meninges are three membranes that envelop the CNS, separating the brain and spinal cord from the body cavities.◦ Dura matter

Outermost layer, hardest, toughest, most fibrous◦ Arachnoid

Middle membrane, less dense, web like in appearance

◦ Pia matter Innermost, thin, compact, very vascular

Supplies blood for the CNS tissue

**Inflammation of the meninges is called meningitis

The brain contains about 100 billion neurons.

The brain in the dog and cat is a highly evolved structure but is less well developed than in more specialized mammals, such as primates.◦ The brain of a canine is more immature than the

human brain at birth, but maturation of cerebral function proceeds at a higher rate. A 1 month old puppy has near adult development of

the CNS

The brain is divided into ◦ Forebrain

Motor area Controls gross, fine, and complicated muscle movements Collects, recognizes, and organizes sensations of pain,

temperature, touch, position, and movement

◦ Midbrain Located between the forebrain and hindbrain

Contains auditory, visual, and muscle control centers and is involved in body posture and equilibrium

◦ Hindbrain Reflex centers for controlling respiration and circulation

Damage to this area instantly causes death